I believe it to be an unconscious choice for a person to be gay. Just like every thing in life, it's your choice whether you like it or not, this works the same way. It's a choice that you do not realize you are making, but you make anyway. If this is true, it would make sense to say that certain people adapt to everything, and are therefore bisexual. If it is a person's choice, then it is perfectly normal for it to happen.
Apparently, 10% of the human population is gay or bisexual (gay can be for males and females btw.).
Turns out that the 10% fact is false, according to wikipedia. See here.
Unfortunately, in modern society, being gay or bisexual isn't quite accepted yet. A discussion like this could very well lead to arguments of whether it is O.K to talk about this or not.